I think this might be putting the cart before the horse. It doesn't really hit the multiple ways Discord is used. From friend groups, to organized gaming, to software support services. Moving you and a couple friends is easy. However as the group grows larger it becomes harder to move.

I think a better question right now is "what does Discord consider adult content?" Is it just NSFW stuff or does it include other topics as well? If everyone's account becomes a flagged as a teen account and the content isn't NSFW then what is the reason to leave at that point? You aren't forced to show an ID and face scan to keep using the platform. So most servers may be able to just keep business as usual.

It's FAR more burdensome to assemble some amalgamation of features and content to be a replacement and move the community over. At least at the moment. Seems very reminiscent of the first exodus to Mastodon. Where there was excitement in the beginning but ultimately people went back to Twitter and stopped using Mastodon because only the most hardcore privacy nerds moved.